Method of making nodular iron

ABSTRACT

IMPROVING THE GRAPHITE NODULE COUNT AND REDUCING THE DEGENERATE GRAPHITE SHAPE IN THE PROCESS OF MAKING NOCULAR CAST IRON BY ADDING GERMANIUM, LEAD, SALTS OF THE TWO MATERIALS OF COMBINATIONS OF ALL, SO THAT THE AMOUNT OF LEAD OR GERMANIUM OR COMBINATIONS THEROF FALLS IN THE RANGE OF .002% BASED ON THE WEIGHT OF IRON.

Aug. 10, 1971 w, MOORE ET AL 3,598,576

METHOD OF MAKING NODULAR IRON Filed Aug. 13, 1968 4 Sheets-Sheet 1 FIG. 2

INVENTORS W/LL/AM H. MOORE -/-/AR/?) H. KESSLER BY M 4, new! I XTTORNEYS Aug. 10, 1971 w, MOORE ET AL 3,598,576

METHOD OF MAKING NODULAR IRON 4 Sheets-Sheet 2 Filed Aug. 13, 1968 FIG. 3

FIG.4.

INVENTORS WILL/AM H. MOORE HARRY H. KESSLER Aug. 10, 1971 w, MQQRE ET AL METHOD OF MAKING MODULAR IRON 4 Sheets-Sheet 5 Filed Aug. 13, 1968 FIG. 5

FIG. 6

INVENTORS WILL/AM H. MOORE HARRY H. KESSLER CENTIGRADE Aug. 10, 1971 Filed Aug. 13. 1968 w. H. MOORE ET AL 3,598,576

METHOD OF MAKING NODULAR IRON 4 Sheets-Sheet L Ta mi I l I I I I0 2.0 3.0 4.0 5.0

% c 2% SILICON FIG 7 INVENTORS WILL/AM H. MOO/PE HARRY H. KESSLER United States Patent Ofice 3,598,576 METHOD OF MAKING NODULAR IRON William H. Moore, Purchase, N.Y., and Harry H. Kessler, Clayton, Mo., assignors to Meehanite Metal Corporation Filed Aug. 13, 1968, Ser. No. 752,249 Int. Cl. C22c 37/04, 37/10 U.S. Cl. 75-130 3 Claims ABSTRACT OF THE DISCLOSURE Our invention relates to nodular cast iron in general and, more particularly, to the production of nodular iron having nodular graphite of smaller size and more rounded sha e.

I t is well-known by those skilled in the art, that cast iron, with nodular graphite caused by the presence of magnesium or cerium, or both, may contain a relatively large number of small nodules or a smaller number of large nodules. These nodules may be truly round or spheroidal in shape or they may be ragged in outline, or mixed with graphite of the vermicular type, or compacted flake graphite, or cellular type graphite, or exploded nodules, all of which varieties may be expressed by the single term degenerate graphite shapes. It is also well-known, that the presence of such degenerate shapes in the nodular iron structure leads to inferior mechanical properties, particularly with regard to such properties as impact strength and elongation.

It is also well-known that the presence of smaller, more numerous, well-rounded nodules, lead to improved tensile and yield strength and greater freedom from graphite segregation in the casting, sometimes expressed by the term carbon flotation.

An object of this invention is to provide a means of producing smaller, more rounded graphite nodules. A further object is to provide means of producing improved mechanical properties in the nodular cast iron. A still further object is to provide a nodular cast iron exhibiting improved graphite structure in heavier casting sections. Another object is to provide means of overcoming the effect of certain tramp or trace elements in the composition of the nodular cast iron. Another object is the production of a nodular cast iron with improved uniformity of nodular graphite shape and size throughout the casting section and free from the problem of carbon or graphite flotation. other objects of this invention will become apparent from the specifications and drawings in which:

FIG. 1 is a photograph at 100 diameters of the structure of a nodular cast iron exhibiting an inferior graphite shape.

FIG. 2 is the photograph of a typical nodule in this same iron at 600 diameters.

FIG. 3 is a photograph at 100 diameters of the same nodular cast iron, to which an addition of lead has been made.

FIG. 4 is a photograph at 100 diameters of the iron of FIG. 2 to which an addition of germanium has been made.

FIG. 5 is a photograph at 100 diameters of a nodular cast iron containing .056% aluminum as a tramp element.

FIG. 6 is a photograph at 100 diameters of the nodular cast iron of FIG. 5, to which an addition of .02% lead has been made.

3,598,576 Patented Aug. 10, 1971 FIG. 7 is a drawing of the 2% silicon section of the iron carbon silicon diagram and superimposed thereon the melting point of the oxides of various elements often found in nodular cast iron or added to nodular cast iron as trace elements.

The effect of tramp elements often called subversive or deleterious elements on the graphite form of nodular cast iron has been recognized ever since the discovery that certain elements such as magnesium, cerium, calcium, yttrium and others, when added to cast iron in certain critical amounts, would lead to the occurrence of graphite in nodular or spherulitic shape in such cast iron. The manufacture of nodular cast iron has been the subject of numerous patents and patent applications for the past 20 years.

It has been recognized by those skilled in the art, that the exact shape of the nodules, freedom from degenerate graphite, and the size of the nodules or nodule count is influenced by a large number of factors such as-carbon equivalent of the metal bath, the rate of cooling of the cast iron, the presence or the absence of certain tramp elements, and the chill value of, or graphitizing power of the bath. For example, a molten metal of high initial chill value will generally lead to larger nodules, fewer in number and of poorer shape than a molten metal of lower chill value would, under the same conditions of manufacture. Altering the chill value of the bath to a lower value, by late inoculation with silicon (either before or after a nodularizing element has been added), will generally increase the nodule count and improve the nodule shape. Similarly, a bath of higher carbon equivalent value, for instance a hyper-eutectic composition, will lead to improved nodule count and improved shape than would a bath of more hypo-eutectic compositions. Additionally, it has been recognized that hyper-eutectic bath compositions tend to produce castings with more tendency for carbon flotation than the hypo-eutectic bath compositions, under the same conditions.

In our invention we have also recognized the fact that the presence of absence of certain elements, in minor quantities, may mask, neutralize, or negate the previously mentioned influence of bath composition or bath chill value.

Certain elements, notably titanium, lead, aluminum, and others, have been recognized as being potent factors in the presence of degenerate graphite shapes, particularly in heavier, slower-cooling sections of nodular cast iron. To counteract their effect, it has long been the practice to include cerium in the nodularizing addition. Such cerium may be added at any stage of manufacture, but is generally considered more effective as a late addition.

Other elements, such as-:tin, arsenic, bismuth, antimony, and others, have been shown by various experts to act, under certain conditions, in the direction of improved nodule count and improved nodule shape. Most of these elements, when added in too great a quantity, will reverse their role of graphite improvement and will lead to degenerate graphite shapes. Elements, such as bismuth, while improving nodule count, must be used in conjunction with cerium, so as to avoid the production of degenerate shapes.

Considerable confusion exists as to the exact amounts of these various elements that may be helpful under one set of conditions or harmful under a second, but different, set of conditions. This confusion is due, in part, to the large number of elements that may be influential, the difliculty of the exact chemical determination of these elements, the cumulative eifect of various combinations of elements which may be additive or subtractive and, in general, to the lack of knowledge of the exact mechanisms that apply during the solidification of graphite in the nodular form.

As a typical example, cerium is regarded by many of those skilled in the art, as a harmful element, while an equal number of skilled metallurgists regard it as an element that is advantageous in that it Works in the direction of neutralizing the harmful effect of other elements.

The truth of the matter lies somewhere in between. Cerium, in small quantities, under certain conditions, will be quite beneficial, while in larger quantities and under other and different conditions, may act in the direction of producing degenerate graphite shapes. The exact same behavior may be attributed to magnesium in its role as a nodularizing agent. Small quantities of magnesium do not produce completely nodular structures.

It is generally recognized that the presence of at least .03% or .04% is necessary to produce completely nodular structures. Under other conditions as little as .005 or .01% of magnesium may be fully effective. On the other hand, larger amounts of magnesium, say about .08% or more, tend to produce less perfect graphite shapes under certain conditions.

In general, all of the common tramp, subversive or deleterious elements will, under any given set of conditions, produce a beneficial effect in small quantities and will produce a harmful effect in larger quantities; thus, forecasting the behavior of these trace elements, under a given set of conditions, is an extremely difficult, if not impossible, matter. While a general qualitative effect may be forecast, it is not possible, at this stage of skill in the art, to make any quantitative forecast.

We have made the unusual discovery that certain low melting point elements, particularly lead and germanium, act in a manner to produce an increase in nodule count and improved graphite shape, when added to nodular cast iron. This discovery is unusual, in view of the fact that little or nothing is known about the behavior of germanium in cast iron and that lead has always been regarded as a very harmful element, in that it leads to the presence of degenerate graphite in nodular iron castings. Experts have proposed the use of cerium in any nodular cast iron, to offset the harmful effects of lead. We find the reverse to be true, and will often include lead in a nodular cast iron composition, to negate harmful effects of cerium. Up to the time of this disclosure, lead has been universally regarded as a subversive element, to be avoided at all costs and under all conditions.

We have discovered that other elements, such as his muth, antimony, tin, arsenic, selenium, and so on will also act in the same direction as lead and germanium, under certain conditions of manufacture.

We have also found that the silicates, or oxides, of these r elements, are effective in promoting this effect, although perhaps under most conditions, are not as effective as the metals themselves.

We have found the use of germanium, or lead, to be particularly effective, in that they produce a greater improvement in nodule count, particularly when present in the iron in the range of from .002% to .02%. We do not wish to limit ourselves to an exact range of composition, because of the difficulty of accurate chemical determination of these elements and, indeed, of all trace elements. Thus, there may be certain conditions, where even smaller amounts of lead or germanium may be effective and there may be other conditions where still larger amounts may be used effectively. As many of these elements may, on the one hand, augment the effect of lead or germanium or, on the other hand, may tend to counteract the effect of lead or germanium, it may be realized why it is so difficult to be exact about the amount of lead or germanium, singly, or in combination, that may be effective in producing the product of our invention.

We do not know exactly why lead or germanium act to produce the effect of improved nodule shape and improved nodule count, but we consider the fact that they produce low melting point inclusions or compounds and the fact that they --are adjacent or sister elements in the periodic table of elements, to be significant. We have, in general, been able to develop a working theory, to explain the mechanism of our invention and, as a result of our discovery, have been able to generally enlarge our invention, to cover the use of a great number of elements, singly, or in combination, and make a tremendously important contribution to the state of the art. Naturally, we do not wish to be limited to an exact theory, because of the quantitative difficulties we have already discussed.

There is no universally acceptable theory regarding the mode of formation of graphite nodules or spherulites in the solidification of cast iron to which effective amounts of nodularizing agents have been added. It is generally considered that nodular graphite precipitates on a nucleus, such as magnesium silicide, in the case of magnesium nodular iron, and that, with the exception of hypereutectic nodules, solidification starts with the crystallization of austenite and nodules, with the austenite surrounding the nodules and with nodule growth proceeding by migration of carbon in the austenite to the graphite nodule. The absence of other well known nuclei, such as manganese sulphide, is essential to the formation of nodular graphite, which is the chief reason why sulphur has to be reduced to a very low level, before nodularization will be effective.

As with all solidfication of molten metal, it is apparent that the presence or absence of certain nuclei in the melt, is an integral part of crystallization. In the absence of nuclei, which might prevent undercooling with rapid and simultaneous crystallization of graphite nodules and austenite envelopes, the size and shape of the nodules will largely depend on the nuclei number and the linear velocity of crystallization. With a high nuclei number, resultant nodules will be extremely small and with a high linear velocity of crystallization they will tend to be well-formed.

When other nuclei, usually present as inclusions, exist in the melt, they may very materially affect the very delicate balance necessary for the formation of graphite nodules and even when they do not suppress this formation entirely, they may nevertheless affect the nuclei number and the linear velocity of crystallization, to the point where nodules are less numerous and may be imperfectly formed or may be formed so slowly they tend to float in the melt and produce extreme segregation, also known as carbon flotation.

The exact identification of the composition of the common inclusions in cast iron is a matter of extreme difliculty. In the first place, common laws regarding chemical inter-reactions do not always apply at the elevated temperature that exists in molten cast iron undergoing the process of solidification. It is generally recognized that these inclusions take the form of oxides, silicates, sulphides, and nitrides of various elements and, more correctly, as complexes of oxides and sulphides. It is also known that these complexes may have high melting points and be insoluble in liquid cast iron, thereby precipitating before solidification or they may have lower melting points and be soluble or miscible with liquid cast iron. Their temperature of formation and their relative solubility in the melt will, in a large manner, control their general effect on solidification and their effect on the precipitation of nodular graphite from the melt.

For the most part, we regard the silicate complexes of manganese, aluminum and titanium as being present in most cast irons and as exerting the greatest influence on the process of solidification. In the case of the manganese silicate-sulphide complex, for example, it is known that either the elimination of sulphur or of manganese will allow the formation of nodular graphite or, more correctly, the suppression of the more common flake graphite. In the case of titanium complexes, it is known that the presence of titanium tends to result in the formation of undercooled type D graphite, rather than the more conventional flake types of graphite. The role of aluminum or alumina complexes is not as well understood, but as most refractories and most ferro alloys and inoculants contain quantities of aluminum, it is extremely diflicult to produce a melt free from alumina complexes and aluminum-bearing inclusions.

In the process of our invention we have found that in high purity melts, in particular, elements likely to be present and producing high melting point inclusions, e.g. aluminum, cerium and titanium, and even magnesium, in larger quantities, tend to produce inferior and degenerate graphite shapes. We have, in FIG. 7, expressed the melting point of the oxide of the element as a standard of comparison, because very little data is available regarding the melting point, the composition, or the solubility and stability of the silicates or silicate-sulphide complexes of these elements. We proceed on the general assumption that the elements with high melting point oxides tend to have higher melting point silicates or silicate complexes. We realize, also, that for any combination of elements, the exact melting point is a question of conjecture and that all ceramic combinations tend to exhibit certain lower melting point eutectic combinations.

On the other hand, we have discovered that elements which tend to form lower melting point oxides and, therefore, lower melting point silicates and silicate complexes, tend to help promote the formation of smaller and more perfectly formed nodular graphite shapes, although, by themselves, they are not nodularizing elements. It is our contention, at least for the mechanism of our invention, that the elements having lower melting point oxides, tend to combine with those elements having higher melting point complexes tending to produce inferior nodule shapes and to render them ineffective by lowering the overall melting point and, perhaps, even the solubility of these complexes in the melt.

Thus, we are able to effectively utilize elements like lead, as shown in FIG. *6, to neutralize the harmful effects of aluminum, shown in FIG. 5. We consider that it is due to lead forming with aluminum a low melting point silicate complex, which effectively prevents any effect it may have on the crystallization process for nodular graphite. It is also reasonable to suppose that excess quantities of lead or other elements forming low melting point complexes, or inclusions, may tend to act with magnesium nuclei present in nodular cast iron, e.g., magnesium silicate, and thereby impair their effectiveness in producing complete nodularity.

Sufiice it to say that, in the process of our invention we find that the addition of small quantities of all the elements shown in FIG. 7, having oxides which melt at temperatures below the liquidus-solidus range of nodular cast iron, tend to promote an improved nodular structure. On the other hand, elements that produce high melting point oxides, well above the liquidus-solidus range, tend to promote degenerate graphite shapes and, further, this tendency for producing degenerate shapes may be ofiFset by the deliberate addition of elements forming oxides having a melting point well below the liquidus-solidus range of nodular iron.

In the practice of our invention, we add a sufiicient quantity of low melting point oxide element, such as lead, to offset the quantity of higher melting point oxides like aluminum, which may be present in the melt; thus, for example, if aluminum is present in the amount of .05 in the melt, we will add between and 75% of this quantity in the form of lead. Where titanium, lead and cerium are present, we will consider their combined amounts in determining the amount of lead to add. On the other hand, where elements like arsenic, antimony, bismuth and tin are present, we will subtract them from the amount of lead to be added.

As an example of the process of our invention, We produce a melt containing .02% titanium and .03% aluminum. The arsenic content of this melt was .005 the lead content was .00l%, and tin content Was .002%.

We added to a portion of this melt a magnesium-silicon alloy, to give .04% residual magnesium and found that a cast test bar gave about nodular graphite and about 20% degenerate nodular type graphite, with about 50% of this degenerate graphite present as exploded hypereutectic graphite.

As the total content of high melting point oxide elements was .02+.03 or .05% and the total of the low melting point oxide elements was .005 -|.00l+.002 or .008%, we considered the balance in favor of higher melting point oxides was .05 %.008% or .042%. We, therefore, added 50% of this or .021% as lead to the melt and then added magnesium-silicon alloy, to give .04% retained magnesium to this second melt portion and cast a second test bar. In this case we found that the structure consisted of small nodules and substantially no degenerate graphite or exploded graphite nodules.

We have found that elements, such as-copper and molybdenum, which also for-m lower melting point oxide complexes, tend to Work in the direction of offsetting the degenerate graphite formed by higher melting point oxide materials, but, in general, they are difiicult to judge quantitatively and we, therefore, prefer to use lead and germanium for this purpose. We have also found that elements with low melting point oxides, such as-bismuth, antimony and arsenic, will effectively negate the degenerate graphite formation of higher melting point oxides, but because bismuth is only completely effective when used with cerium and because lead and germanium tend to greatly improve nodule count, we prefer to use them in the practice of our invention.

We are cognizant of the effect of the other elements besides lead and germanium, but rather than deliberately add them as additional agents, we prefer merely to allow for their effect in deciding how much lead or germanium we must add for acceptable results.

Because of the difiiculty of exact chemical determination of the various trace elements, we usually prefer to establish the quantities of elements deliberately added by means of experiments; thus, with a given melt in a given operation, we will add sufiicient magnesium to give a retained magnesium content of .04% and will cast a standard test bar, either of 1" or 3" section, depending on the average section size of castings to be poured. We will then examine the modularity of this sample cast with respect to nodule count, and the presence of degenerate graphite. According to the results of this examination, we will add lead or germanium to the melt, based largely on the quantity of aluminum and titanium present.

We will then cast another test bar with a view to examining the appearance of the structure. On this basis, we try to gauge the minimum amount of lead or germanium addition that will be fully eiTective and try to avoid excessive additions. We then establish this addition as a standard procedure under the particular conditions that exist.

We prefer to limit ourselves to lead or germanium additions, which are essentially less than .05 because of somewhat limited knowledge of the effect of heavier additions, particularly with regard to heavier casting sections. Where such an addition is not effective, we prefer to adjust melting conditions or charge materials, so as to lower the need for heavier additions of germanium or lead. For example, we work towards lowering the aluminum or titanium content of the melt.

As far as a choice between lead and germanium are concerned, we favor lead, because of its lower cost, but we prefer to use germanium where it is important to produce a more ferritic structure. We find that germanium has a propensity to form greatly increased quantities of as-cast ferrite.

We do not wish to limit our invention to any exact theory or disclosure, but we have described it with a certain degree of particularity and wish to claim:

1. In the process of making nodular cast iron containing small but measurable quantities of aluminum, ti-

tanium, cerium and other elements forming high melting point oxides; improving the graphite nodule count and reducing the degenerate graphite shape comprising the steps of melting the charge and then adding a material selected from the group consisting essentially of lead and germanium to the molten charge, said lead and germanium being characterized by the formation of low melting point oxides, the amount of lead and/ or germanium being in the range of .002% to .02% based on the weight of the metallic charge.

2. In the process of making nodular cast iron; improving the graphite nodule count and reducing the degenerate graphite shape comprising the steps of melting an iron charge and then adding lead or germanium to the molten charge so that the amount of lead or germanium falls between 10% and 75% of the total of the materials of aluminum, titanium, and cerium in the melt, minus the total of the materials of bismuth, arsenic, antimony and tin in the melt.

3. In the process of making nodular cast iron; improving the graphite nodule count and reducing the degenerate 8 graphite shape comprising the steps of melting an iro charge and then adjusting the total of the ingredients of antimony, bismuth, lead, germanium, tin and arsenic so that it is between 10% and 75% of the total of the ingredients titanium, aluminum and cerium, in the melt.

References Cited L. DEWAYNE RUTLEDGE, Primary Examiner J. E. LEGRY, Assistant Examiner US. Cl. X.R. 

